1. Field of the Invention
The present invention relates to integral, brushless permanent magnet drives; and in particular to cooling systems for integral, brushless permanent magnet drives used to power vehicles such as watercraft, aircraft, automotive, commercial and industrial vehicles or other industrial equipment.
2. Description of the Related Art
Most contemporary vehicles are driven by internal combustion engines. These engines convert potential energy stored in a fossil fuel into kinetic energy. As a byproduct of producing energy from combustion of a fossil fuel, these engines tend to generate large amounts of excess heat which can cause the engine to overheat and ultimately fail. In order to keep the temperature of these engines within a safe operating range, these engines are often cooled by an external cooling system.
In the case of automotive vehicles (and many aircraft engines), coolant is pumped through the engine in a fluid conduit. Heat is extracted from the engine into the coolant. The heated coolant is then pumped out of the engine and through a heat exchanger where the heated coolant is cooled by air or an external fluid coolant. In the case of outboard internal combustion engines used in watercraft, the engine is cooled by pumping water from the body of water on which the watercraft is located through the engine. The heated water is then discharged back into the body of water.
Although internal combustion engines and their associated cooling systems have operated effectively for their intended purpose, they also have several drawbacks. During the combustion of a fossil fuel, a majority of the stored energy is lost to heat, friction, or discharged as uncombusted fuel in the exhaust. Typically, internal combustion engines average around 30% efficiency. Secondly, internal combustion engines cannot be used in a bidirectional energy conversion system. Once the fossil fuel has been converted into kinetic energy, the kinetic energy cannot be converted back into a fossil fuel to store the potential energy. Another drawback to using fossil fuels is that a fossil fuel may be accidentally ignited if not handled properly. They are also poisonous to humans and discharge hazardous chemicals into the environment when combusted.
To overcome many of the disadvantages present in internal combustion engines, various electric drives have been suggested either in combination with internal combustion engines (hybrid electric vehicles) or alone to completely replace the internal combustion engine (electric vehicles). Electric drives provide several advantages over internal combustion engines. Electric drives do not discharge harmful exhaust gases into the environment. With the selection of an appropriate battery, the battery is not combustible and does not require direct human contact and thus less hazardous to the user.
An electric drive converts potential energy stored in a battery into kinetic energy when operated in the motor mode. Ideally, these drives also have a second mode in which the drive converts kinetic energy from rotating the rotor into potential energy stored in the battery. Electric drives are particularly well suited for these bidirectional or energy recapture systems because they can reach efficiencies approaching almost 90%. The most common DC drives used in these recapture systems are controlled induction drives, permanent magnet drives, and switched reluctance drives. Controlled induction drives can reach approximately 90% efficiency; switched reluctance drives can reach approximately 92% efficiency; and, permanent magnet drives can reach approximately 94% efficiency. Although the differences in efficiency between these drives appears to be relatively small, even small differences can be quite significant in a bidirectional energy conversion system especially when large integral horsepower drives are used.
As can be seen from the above, brushless permanent magnet drives (“BPMD's”) are capable of the highest idealized maximum efficiency. However, BPMD's have a variety of losses which prevent these motors from reaching this level of efficiency when large integral horsepower motors are used. The losses experienced by BPMD's come from windage losses due to spinning the rotor, friction losses in the moving parts such as the bearings, copper losses due to generating heat in the stator windings, and iron losses due to generating heat in the stator. The two most significant losses in these motors are the iron losses and copper losses. Aside from these direct efficiency losses, in large horsepower BPMD's, the heat generated by these loses can result in an additional parasitic losses due to a decreased magnetic force capacity in the magnets. For instance, in Five (5) horsepower and larger motors, the temperature of the interior of the motor can reach in excess of 100 to 120 degrees Celsius. The stator components such as the varnish coating, wire insulation, etc. can typically withstand temperatures up to approximately 200 degrees C. It is desirable to use high capacity magnets such as Neodymium-Iron-Boron magnets because of their high energy product. However, high efficiency Neodymium-Iron-Boron magnets can withstand temperatures of only 100 degrees Celsius. At temperatures above, 100 degrees Celsius, the magnets begin to irreversibly degrade.
Effective cooling systems are necessary when used on integral horsepower motors. One approach described in the art is the use of a fan attached to the shaft of the motor which either pushes or pulls air through the air gap between the rotor and the stator. These cooling systems provide a cooling airflow to the motor; however, they lack efficiency which is critical in bi-directional vehicle applications. The fan is coupled to the rotor shaft and rotates at the same RPM as the rotor. At low motor speeds or when the rotor is stopped, this may prove to provide insufficient cooling to the rotor magnets. Alternatively, at high speeds, the fan may be spinning too fast and result in cooling the rotor magnets more than is necessary.
Another solution known in the art is to use an external cooling system. For instance, U.S. Pat. No. 5,939,808 to Adames discloses a motor having a rotor, a stator, and a drive housing. The drive housing includes an independent cooling system including a conduit embedded in the housing and connected to a heat exchanger. Cool fluid is pumped through the housing and heated. The heated fluid is then returned to the heat exchanger and cooled down before returning to the drive housing.
Although having an external cooling system has advanced the art, even this device has drawbacks. For instance, this cooling system does not have an independent control. Although a separate pump is disclosed, the pump appears to be operated at a constant speed much like the coupled fans discussed above. In addition, the device described in the '808 patent, does not have a feedback loop to provide for a closed-loop control. Accordingly, this device has many of the same drawbacks found in the fans coupled to the rotor. The cooling system may either overcool the rotor magnets and waste energy or under cool the rotor and risk irreparably degrading the magnet quality. In any event, the cooling system does not have any way to determine the amount of cooling needed. Another drawback to this device is that cooling conduits are located in the housing. Aside from the manufacturing difficulties this presents, this location distances the maximum cooling capacity from the rotor and rotor magnets, which are the most temperature sensitive components in integral brushless permanent magnet drives. In order to provide an adequate amount of cooling to the rotor magnets utilizing this system may require excessive operation of the cooling system to compensate for being distanced from the rotor magnets.
Accordingly, it would be desirable to have a high efficiency integral electric drive. It would also be desirable to have a high efficiency integral drive capable of high torque without magnet degradation due to overheating. It would be further desirable to have a cooling system for an electric drive which is configured to cool the rotor magnets. It would be still further desirable to have a cooling system for an electric drive which has a control which is independent of the motor operation.